Gordon Brown and Tony Blair face being questioned in public over their roles in the run-up to the Iraq war after the chairman of the independent inquiry indicated that he is to summon the prime minister and his predecessor to give evidence.

In a setback for Brown, who had hoped the inquiry would be held in private, Sir John Chilcot has ruled that all witnesses will be expected to give evidence in public. This will apply across the board unless there are "compelling reasons" in a small number of cases for evidence to be heard in private. This would be if evidence could jeopardise national security or upset allies.

The decision by Chilcot opens up the prospect that Blair and Brown will be cross-examined on their roles in the Iraq war during the build-up to the general election that is expected to take place next year. Chilcot is not giving any indication on the timings of his hearings, which means that he could defer politically sensitive appearances by Brown and his predecessor until after the election.

The move to open up his hearings, which came on the eve of a Commons debate tomorrow on the inquiry, shows that a wholesale change of the terms has been carried out since the inquiry was established by the prime minister last week. The decision to summon Brown and Blair for public hearings was disclosed by Nick Clegg, the Liberal Democrat leader, who met Chilcot today on privy council terms. Chilcott held a separate meeting with David Cameron on the same terms.

In a letter to Chilcot, Clegg wrote: "I was pleased to see how much progress has been made from the initial position set out by the prime minister last week regarding the process of the inquiry … It was also good to hear you confirm that you will be seeking evidence from Tony Blair and others in high office at the time, and would want their evidence to be held in public except in very limited circumstances."

Cameron, who asked Chilcot to summon Brown before the inquiry, was more guarded in his remarks. Tory sources said they were "hopeful" that the prime minister would be summoned before Chilcot.

The prospect of public grillings for Brown and Blair shows how the prime minister's plans for the inquiry have been dramatically changed since it was established last week. On 15 June Brown told MPs that the inquiry would be modelled on the Franks inquiry into the 1982 Falklands war, which met in private. He said: "I believe that that will also ensure that evidence given by serving and former ministers, military officers and officials is as full and candid as possible."

The prime minister announced last week that Chilcot would chair the five-strong committee of non-political privy counsellors. Clegg said that Chilcot had agreed to a series of other changes:

• Expert assessors, including retired senior military officers, public and constitutional law experts and experts on post-war reconstruction, will support the five members of the inquiry. These experts could cross-examine witnesses.

• Chilcot has indicated that he remains open to the idea of publishing an interim report, according to Clegg. The Lib Dems and Tories want an interim report to be published before the general election. Chilcot, whose inquiry is not due to report until July 2010, is expected to be highly cautious about publishing on the eve of the election.

• Witnesses will not, as expected, be required to swear an oath. But Clegg says Chilcot has indicated that he will specify to witnesses in writing and verbally that their evidence must be truthful.

The signals from Chilcot – likely to be accepted by the government, which has given the former Northern Ireland Office permanent secretary a free hand – should reduce the chances of a defeat for No 10 in tomorrow's Commons debate.

Government sources said they were relaxed with Chilcot's plans. "We have always said we would fully co-operate with the inquiry," a senior source said.